Keynote, well known for its application performance testing and monitoring services, just announced a new version of its KITE (Keynote Internet Testing Environment) that is now capable of testing Web 2.0 sites that make use of AJAX, Flash, and other "hidden" methods of obtaining content.

Ensuring that the additional burden placed on servers by "hidden" requests is imperative when attempting to understand both capacity and end-user performance.

But when you're laying out that testing plan, don't forget about any APIs you might be providing. Like Twitter, Google, Facebook, and Amazon, if you're using an API to allow integration with other Web 2.0 sites or applications make certain you performance test that, as well. And then test them at the same time you're load-testing your application. As we've learned from Twitter's most public stability issues, the APIs are going to put additional stress on your network, servers, and databases that must be considered when determining capacity and performance levels.

Important, too, is to take into consideration RSS feeds coming from your site. Include those in your performance test, as the retrieval of those files adds an additional burden on servers and can impact the performance of the entire site. Most feed-readers and aggregators poll for RSS feeds on a specified interval so set up a script to grab that RSS from multiple clients every 10 minutes or so during the testing to ensure that you're simulating maximum load as closely as possible. 

Performance testing sites today is getting more complex because we're adding so many entry points into our sites and applications. Don't forget about those extra entry points when you perform your load-testing or you might find out in a most public way that your application just can't handle the load.